1. Field of the Invention
The present invention relates to a gradient device for changing, over time, a composition of a mobile phase sent through an analytical flow path of a liquid chromatograph, and particularly to a low-pressure gradient device.
2. Description of the Related Art
As a liquid chromatograph, there is one including a low-pressure gradient device for supplying a mobile phase to an analytical flow path while changing its composition over time (refer to Japanese Patent Application Laid-Open No. 5-312795, for example). In such a low-pressure gradient device, generally, a plurality of kinds of liquids are prepared and times when the liquids are suctioned into a reciprocating pump, such as a plunger pump, are controlled to adjust a mixing ratio of the liquids to thereby change the composition of the mobile phase.
FIG. 6 shows an example of a prior-art low-pressure gradient device.
There is a plunger pump 10 for suctioning and discharging liquids by sliding of a plunger 8 in a pump head 6, an inlet flow path 54 is connected to a liquid inlet of the plunger pump 10, and an outlet flow path 12 is connected to a liquid outlet. The plunger 8 is driven to reciprocate on a straight line by a driving portion 16 including a motor and a cam mechanism, and operation of the driving portion 16 is controlled by a control section 58.
As the mobile phases to be sent by the plunger pump 10, four kinds of mobile phases A to D are prepared. Suctioning flow paths 52a to 52d having one of their ends for respectively suctioning the mobile phases A to D have the other ends meeting each other at a meeting portion 55 and connected to the inlet flow path 54. A switching mechanism 53 formed to open only one of the suctioning flow paths 52a to 52d is provided. The switching mechanism 53 includes opening/closing valves 53a to 53d provided on the respective suctioning flow paths 52a to 52d and allows only one of the mobile phases to be suctioned into the plunger pump 10. Operation of the switching mechanism 53 is controlled by a control section 58. Although it is not shown in the drawing, the inlet flow path 54 and the outlet flow path 12 are respectively provided with check valves, the inlet flow path 54 is open and the outlet flow path 12 is closed during a suctioning operation of the plunger pump 10, and the outlet flow path 12 is open and the inlet flow path 54 is closed during a discharge operation of the plunger pump 10.
In the gradient device, two of the four kinds of mobile phases A to D, for example, are successively suctioned into the plunger pump 10 and mixed by switching the flow path at a predetermined time, and the mixed liquid is sent into an analytical flow path of the liquid chromatograph. By changing times when the mobile phases to be suctioned into the plunger pump 10 are switched over time, the composition of the mobile phase to be sent into the analytical flow path is changed.
Although the mobile phases to be suctioned into the plunger pump 10 come in contact with each other at the meeting portion 55 in the above-described gradient device, only the suctioning flow path of the mobile phase to be suctioned into the plunger pump 10 is open and the suctioning flow paths of the other mobile phases are closed, and therefore, another mobile phase does not flow back into the closed suctioning flow paths. However, it was found that the mobile phase having a higher density flows into (flows back into) the flow path for the mobile phase having a lower density, if there is a difference in density between the mobile phases in contact with each other. If the mobile phase flows back, the mobile phase that should be suctioned into the plunger pump 10 flows into the flow path for the other mobile phase, and therefore, a predetermined quantity to be suctioned cannot be achieved, which affects the composition of the mobile phase sent from the plunger pump 10, and reproducibility of an analysis result of the liquid chromatograph cannot be obtained.